Traditionally, in the field of optical inspection of integrated circuits wafer bumps, 2D inspection was carried out first and then 3D inspection was carried out after the first inspection. In this traditional embodiment, the 2D camera system works in a free run mode at the maximum possible speed while the scanning speed is kept constant and in function of the camera speed and the optical field of view. Mainly, the 2D subsystem consisted of one of the following. The first system was a TDI type camera employing a constant light source while scanning speed is tightly controlled so that the resulting image on the TDI sensor travels across the sensor at the sensor shifting speed enabling the light integration over multiple sensor line. This synchronous speed allows, depending on the length of the TDI sensor, much higher sensitivity to the light due to the effective longer exposure (proportional to the number of the TDI sensor lines) while avoiding blurring due to the motion. The second subsystem was an array type camera employing the strobe lighting as the lighting source. The camera controls the strobe by generating the trigger at the end of the previous image transfer and before the next image transfer in a sequence. Provision must be made to store encoder position at the time of the exposure (strobe).
Processing sequentially the 2D and 3D inspections is a source of technical difficulties, such as inaccurate corresponding between 2D and 3D images associated with a same field of view. Besides, proceeding sequentially with the 2D and 3D inspections is a source of important time consumption in the process of wafer bump inspection.